In one form of a fluid pressure control valve, a flow control element, such as a spool or valve member, is movably positioned in a valve chamber between first and second valve positions for selectively fluidically coupling valve ports. The spool member may be directly actuated by a movable armature or plunger. A solenoid controllably positions the plunger, which results in movement of the spool from a neutral position to an actuated position. Typically, a coil spring is used for biasing the spool to the neutral position. Energization of the solenoid coil produces a magnetic force acting on the plunger which is related to a gap between the plunger and a stop. This relationship is represented by a curve in which force is generally inversely proportional to gap. With an on/off type valve, the magnetic force exceeds the opposing spring and flow forces to provide continuous movement of the plunger to a fully actuated position.
With a proportional type valve, it is necessary to control the plunger to stop at intermediate positions. An example of such a proportional valve is shown in Kolchinsky, U.S. Pat. No. 4,790,345. This patent shows a valve housing including a sleeve having first and second tube portions connected by a non-magnetic bridge. The mating surface of the bridge and tube portions are conical. This construction changes magnetic behavior as by modifying the above-described curve to provide constant force during the portion of the stroke at which the plunger is at an axial position corresponding to position of the bridge. As a result, the plunger moves until the magnetic force is balanced with the spring and flow forces so that the plunger stops at an intermediate position. As is known, the magnetic force depends on voltage applied to the coil. Thus, by varying coil voltage the stop position varies. Thus, movement of the spool can be controlled to regulate flow by varying input voltage.
In the case of a pressure control valve, a similar tube is used. However, the force of the pressure differential over the cross section of the spool cross section opposes the magnetic force. Therefore, the larger the diameter of the spool, the larger the required magnetic force.
Aronovich, U.S. Pat. No. 5,299,600, owned by the assignee of the present application, discloses an analog proportional pressure control three-way valve. The three-way valve disclosed therein uses a pressure dividing circuit that divides regulated pressure between an outlet and end port opening in a regulated position to create an intermediate pressure to balance force from the solenoid. For use in high flow applications, such a valve spool needs a large spool diameter. However, control pressure acts on the entire cross-sectional area of the valve, requiring larger magnetic forces and thus solenoid coil size.
The present invention overcomes one or more of the problems discussed above.